Portable Analyzers Find PIM At Cell Sites

Passive intermodulation (PIM), once relatively unknown in the RF/microwave industry, has become almost a household term when describing the performance of modern wireless communications systems. As its name suggests, PIM is signal distortion that results from the mixing of multiple signals, often in a loose connector or in a rusty termination within the wiring of a cellular communications tower. And because so many modern wireless communications formats are based on multiple carriers, PIM can plague the infrastructure of cellular towers and repeaters. This often occurs in hard-to-reach places, raising the system noise floor and resulting in dropped calls, access failures, and low data rates in wireless systems.

1. The MW82119A family of PIM Masters portable instruments can locate PIM sources at cellular and other remote wireless communications sites.

Fortunately, the latest additions to the PIM Master™ line of portable analyzers from Anritsu Co. make remote, on-site PIM testing not only possible but practical. They provide all of the measurement capability needed for PIM measurements in portable units that fits into a backpack and operate for several hours on a single battery charge.

The MW82119A line of PIM Master portable analyzers (Figs. 1-3) are small, battery-powered instruments with a highly visible, 8.4-in. (213-mm) touchscreen display for ease of use and visibility under a host of operating conditions. The product family includes different models for specific communications bands, such as 700 MHz for testing Third-Generation (3G) Long-Term-Evolution (LTE) cellular systems and 1900 MHz for checking PIM in Personal Communications Services (PCS) wireless equipment (see table).

These new, lightweight portable test instruments build on the firm foundation established by the company’s Site Masters—first introduced in 1997—and more recently its Spectrum Masters, with as much as 95-dB dynamic range from 100 kHz to 6 GHz. In addition, the models MW8208A, MW8209A, and MW8219A PIM Masters introduced last year were also aimed at specific wireless/cellular bands, such as 869 to 894 MHz and 1930 to 1990 MHz.

Weighing less than 27 lbs, these battery-powered analyzers easily bring PIM test capability to remote locations, such as the top of cellular towers where RF/microwave transmit/receive electronics are often found. The new MW82119A PIM Masters measure just 13.8 x 12.4 x 6.0 in. (350 x 314 x 152 mm) and will run for as long as 2.5 hours on a single battery charge. Depending upon model, they weigh between 20 and 27 lbs (9 to 12 kg) for ease of transport to a wireless communications site and even up a tower. Since there is typically no AC power available at the top of a cellular tower, the completely self-powered, self-contained operation of the MW82119A PIM Masters makes them ideal for such remote measurements.

2. The battery-powered MW82119A PIM Masters are small and lightweight enough to bring to the top of a cellular communications tower.

Characterizing a communications system and its cables and connectors for PIM requires multiple test signals at sufficient power levels. The MW82119A PIM Masters deliver, boasting a pair of test tones that can each be set at levels from +25 to +46 dBm output power (0.3 to 40.0 W output power). And as cellular/wireless services expand, the number of towers and wireless sites increases. Having multiple service providers operating from one site often requires the use of distributed antenna systems (DAS) with multiple coaxial connections that can serve as PIM generation points. In addition, any metal structures located near these antennas can also encourage the generation of PIM.

The battery-operated MW82119A PIM Masters are self-contained test systems that do not require an additional external controller. Each model generates two continuous-wave (CW) test tones in a specific wireless transmit band for injection to an antenna feed network and then measures the magnitude of the third-, fifth-, or seventh-order PIM products that fall into the corresponding receive band. They can produce test tones at levels from +25 to +46 dBm in 0.1-dB steps and feature a PIM measurement range of -130 to -70 dBm. Test tones are available at a 7/16 DIN female RF output connector.

The MW82119A PIM Masters feature a host of useful automated measurements, including the firm’s patented Distance-to-PIM™ measurement capability which helps to isolate the source of PIM in a communications tower or system, as well as in lower-power DAS equipment. A user simply sets a handful of starting parameters, including the two carrier frequencies (F1 and F2); the intermodulation order (such as the 3rd, 5th, or 7th intermodulation products); the output test power level; the test duration (from 1 to 3600 s); and lower and upper limit lines. For analyzers equipped with the Global-Positioning-System (GPS) option, that feature can also be turned on or off. For Distance-to-PIM measurements, the distance and the velocity of propagation for the cable under test must also be entered.

3. The MW82119A series of PIM Masters includes models for testing wireless frequency bands from 700 to 2100 MHz (see table).

Additional automated tests include PIM-versus-time and swept PIM measurements. For PIM-versus-time measurements, the two carrier frequencies F1 and F2 are fixed while the PIM magnitude is measured as a function of time. This measurement, which provides a visual indication of PIM stability with time, is ideal for dynamic PIM testing and for peak PIM measurements for pass/fail testing. When performing swept PIM measurements, carrier frequency F1 can be fixed and frequency F2 swept, or carrier frequency F2 can be fixed and frequency F1 swept, with the results showing PIM magnitude as a function of frequency (and also showing the worst-case PIM level for a system).